The objective of this project is the development of an understanding of how a specific maternally-produced component of the egg cytoplasm, in response to the X:A chromosome balance, regulates the expression of an X-linked gene that controls sex determination and dosage compensation (X-chromosome transcription rate). It is believed that the work will be directly relevant to the question of how cells become determined during embryonic growth. Work planned for this year includes a test of the hypothesis that the Sex-lethal gene becomes committed to activity or inactivity during the first few hours of development, though it (continues to) functions much later (how much later, to be determined). The genetic organization of the Sex-lethal locus will be investigated through an analysis of a wide array of single and double mutants in this gene. In addition, the number and proximity of neighboring vital genes will be determined. Finally, attempts will be made to characterize the maternally-synthesized egg factor that is necessary for the activation of the Sex-lethal locus.